PART II. POLAR MAGNETIC PHENOMENA AND TERRF.LLA EXPERIMENTS. CHAP. V. 639 



It would be interesting to find out whether the pretty results obtained by Bredichin in his mecha- 

 nical investigations of comets' tails could be made to harmonise with the theory of electric discharges 

 through rarefied gases. The formation of several distinct tails from one comet would then possibly have 

 causes corresponding more or less to those of the formation of the various distinct pencils of cathode- 

 ravs in an electric or magnetic field (cathode-ray spectrum). 



It is now generally assumed that comets belong to our solar system, because no comet has an 

 undoubted hyperbolic orbit. This also agrees with the fact that the spectra of comets exhibit on the 

 whole a great similarity. 



In a subsequent article we shall see how our theory of an electric radiation of matter from the 

 sun can give a satisfactory explanation of the comet's formation, even when its orbit carries it to a 

 distance of 1000 or 10000 astronomical units from the sun. 



125. Halley's Comet, May, 1910. An exceptionally favorable opportunity of testing the views 

 here brought forward regarding comets' tails presented itself in May, 1910, when Halley's comet crossed 

 the sun's disc at so comparatively short a distance from the earth that there was a possibility of the 

 earth's passing through the comet's tail. When a magnet as great as the earth came into the comet's 

 tail, there would surely be magnetic effects to be observed upon and from the tail, if the latter con- 

 sisted of some kind of electric corpuscle-rays. 



It was Herr KROGNESS, who, happening to read in an astronomical journal that Halley's comet 

 would come so near to the earth, suggested that we should go up to my observatory on Haldde Mt. for 

 the purpose of studying the possible effects of the passage. This was arranged, when I had succeeded 

 in getting a friend of mine, Herr SCHIBSTED, to share the expenses equally with me. 



In order to secure a more widespread interest in these observations, I sent out, in March, 1910, 

 the following circular to a number of observatories and a few periodicals (e. g. 'Nature', April 21). The 

 figures that were reproduced in the circular are here omitted, the reader being referred to the same or 

 better figures already printed in the present work. 



"I beg to direct your attention to the following. 



"It is my intention, at Kaafjord in Finmarken (in the N. of Norway), together with my Assistant 

 Mr. O. KROGNESS, to take magnetic and atmospheric observations during the period yth May to ist June 

 lext in connection with the transit of Halley's comet across the sun's disc on the i8th igth May. 



"The thing is, that it is conceivable that the tail of the Comet may chiefly consist of electrical cor- 

 puscular rays, and, if this be so, we would expect that these rays, owing to Earth magnetism, would 

 be drawn in, in the Polar regions, in zones analogous with the Aurora zones, assuming the tail of the 

 comet to be of sufficient length to reach the Earth. 



" I hese rays will then, in such case, exercise, amongst other things, magnetic influences and elec- 

 tric inductionary effects, especially strong in the Polar regions, and it is particularly such effects we are 

 desirous of tracing. The tail of the Comet, if it should consist, as above assumed, of such radiant 

 matter, will alter its shape at a very considerable distance from the Earth, and we may expect to see 

 similar formations of light to those which occur during my experiments with cathode rays around a 

 magnetic terrella. 



"In my work, "The Norwegian Aurora Polaris Expedition 1902 1903", descriptions will be found 

 in several places of these phenomena, but to elucidate the subject here, I append a few new illustra- 

 tions, which very plainly show the shape of these formations of light. 



"Figures i (217) and 2(218), show how the rays are drawn in, in belts around the magnetic poles 

 ot the terrella, correspondingly, with the Polar-light zones on the Earth. They are taken looking along 

 and perpendicular to the magnetic axis. Fig. t show the spiral rings of light around a magnetic 



